In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC process steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.
Typically, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For example, circuit probe testing is increasingly performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held stationary relative to a vacuum support surface of a prober machine which electrically tests the circuits on the wafer. The prober includes a group of electrical probes which, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits' responses to the excitations.
In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure for the chuck is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.
The chuck can also include a temperature control system which raises and lowers the temperature of the chuck surface and the wafer as required to perform the desired temperature screening of the wafer. It is important to the accuracy of such testing that the temperature of the wafer and, therefore, the temperature of the chuck surface, be controlled as accurately and precisely as possible.
Various approaches to controlling the wafer temperature have been employed. In one prior system, the chuck includes a circulation system through which a cooling fluid is circulated. The cooling fluid is maintained at a constant cold temperature and is circulated through the chuck. Temperature control is realized by activating a heater which is also located in the chuck. The heater is cycled on and off as required to heat the chuck and the workpiece to the required temperature.
This approach has certain drawbacks. A large time lag occurs when heating the chuck because the circulation fluid is always maintained at a low temperature and is always circulating through the chuck. As a result, a large amount of time can be required to heat the chuck and workpiece to a high temperature. Also, the system can be inefficient since much of the energy provided to the heater is wasted in the presence of the circulating cold fluid. Additionally, energy used to cool the fluid is wasted when the chuck and workpiece are being heated.
In another prior system, both a temperature-controlled fluid and a chuck heater are used to control the workpiece temperature. In this system, the fluid is used to bring the workpiece to within a certain tolerance of the desired set point temperature. The heater is then cycled as required to trim the temperature to the set point. This system also suffers from long time lags and poor efficiency.
In still another prior art system, temperature control is implemented using only passive heat transfer to and from a fluid circulating through the chuck. In this system, the chuck is provided with a series of internal channels through which the temperature-controlled fluid is circulated. The chuck temperature is controlled by controlling the temperature of the fluid. This system also suffers from long time lags and relatively low efficiency.
In some applications, such as where a wafer is being tested on a circuit prober, it is important to reduce the electrical noise introduced into the system, since such noise can adversely affect the measurements being made by the prober. Introduction of noise into the measurement is a common problem where temperature testing of the wafer is being performed on the prober. Power and control signals applied to elements such as resistive chuck heaters are typically in close proximity to the wafer and, therefore, can be substantial sources of noise.
Therefore, it is important that power supplies that provide power to heating elements be as noise-free as possible. As a result, prior thermal chuck power systems include linear power supplies to provide power to heating elements. However, linear supplies tend to be very inefficient. In fact, their power dissipation is highly dependent on input voltage. Therefore, under conditions in which the input line power can vary, substantial inefficiency can result. Also, because the standard European line power voltage level is higher than that used in the U.S., the power dissipation of a linear supply would be higher in Europe than it would be in the U.S., thus requiring different supply and system designs or tolerance of substantial variation in power dissipation. In addition, linear supplies are not capable of power factor correction. Under new European standards soon to be implemented, high-power supplies must be power factor corrected. Linear supplies may not meet these new standards under certain conditions. Therefore, it would be desirable to have a workpiece chuck that is powered by noise-free power signals but does not rely on linear power supplies for power.